biology/iqtree: fix build on arm64

We call the platform "arm64", not "aarch64" (though it seems to be
inconsistent). Tested on powerpc64le, didn't build, as some
headers get confused by the whole "emulate SSE with Altivec" business.

Approved by: portmgr (build fix blanket)
MFH: 2026Q1
Sponsored by: Raptor Computing Systems, LLC